Natural Gas is a vital source of heat energy in the United States. Its selling price is based on volume and heat content. The heat content is greatly influenced by the presence of the heavy (higher molecular weight) components. These heavy components also have a large influence on the gases physical properties, which in turn impact flow rate and volume calculations. The heat content and physical properties of natural gas are primarily determined by calculations based on gas composition.
A common means for determining the gas composition of a Natural gas source consist of capturing a spot sample of the source gas, such, for example, in a pressurized pipeline, transferring the gas sample to a storage vessel where the sample can be stored, then transporting the storage vessel, such as a sample cylinder, to a laboratory wherein it is analyzed by Gas Chromatography.
The constant volume type of sample cylinders, such as the SWAGELOK™ part # 316L-HDFA-300, require purging with sample gas to remove residual contaminates (such as air) prior to filling with sample gas. Most filling techniques require a flow restrictor, which is thermally isolated from the sample cylinder.
The Gas Processors Association (GPA) standard 2166 “obtaining Natural Gas Sample for analysis by Gas Chromatography” requires the use of an extension tube (pigtail) of at least 36 inches in length downstream of the sample cylinder outlet valve. Refer to FIG. 1.
A valve and/or restriction is connected to the end of the extension tube. Said 36″ length of tubing is designed to provide thermal isolation between said valve and/or flow restrictor and the outlet valve of the sample cylinder. Said thermal isolation is required for cylinder purging methods which require rapid depressurization of the sample cylinder gas. Otherwise, when the cylinder is purged with the source gas, the resulting Joule-Thomson (J-T) effect may cause the cylinder outlet valve to cool below the Hydrocarbon Dewpoint temperature (HDPT) of said source gas.
The resulting condensation of the heavier sample gas components render the remaining sample gas unrepresentative of the original gas source. This in turn impacts the calculated heat value, volume, and monetary value of the source gas. When utilizing the extension tube with a valve and/or restrictor the bulk, but not all, of the J-T effect cooling takes place at the outlet end of said extension tube. However, some pressure drop takes place across the outlet valve and depending on its Cv characteristics, causes cooling of the outlet valve.
The pigtail type of thermal isolation is awkward and must be installed and removed from the sample cylinder each time that a spot sample is taken. It is therefore rarely used by the measurement technician for spot sampling. This has been a great source of concern since improper sampling can have a significant impact on assessment of the monetary value of Natural gas.